(i). Field of the Invention
The present invention relates to a method for evaluating a semiconductor substrate. In particular, it relates to a method for evaluating a semiconductor substrate that makes it possible to measure the photoluminescence (PL) distribution of a semiconductor substrate used in a solar cell or the like at high speed and high resolution.
(ii) Description of the Related Art
Production of silicon solar cells has been increasing rapidly. To improve the photoelectric conversion efficiency of the silicon solar cell, it is urgently needed to improve the crystallinity of a silicon substrate used in the solar cell.
To attain high photoelectric conversion efficiency in solar cells, it is essential that carriers (electrons and holes) generated by light irradiation reach electrodes without recombining on the way to the electrodes. That is, it can be said that the conversion efficiency of the solar cell is determined directly by such parameters as the lifetime and diffusion length of the carriers, and a method for evaluating these parameters is needed.
Meanwhile, as causes for recombination of the carriers, crystalline structural defects such as grain boundaries and dislocations and impurity-contaminated parts that exist in a silicon substrate are conceivable. In other words, when crystalline structural defects and/or impurities exist in the silicon substrate, it is expected that the lifetime and/or diffusion length of the carriers are/is shortened, resulting in a decrease in the photoelectric conversion efficiency of the solar cell.
Accordingly, to obtain a solar cell having high photoelectric conversion efficiency, a method for evaluating distributions of crystalline structural defects like dislocations that exist in a silicon substrate and the extent of contamination by impurities as well as the method for evaluating the lifetime and diffusion length of the carriers is important.
As a method for evaluating the diffusion length of carriers (minority carriers in particular), a surface photovoltage method (SPV method) is available. This method is a method which measures the diffusion length of the carriers of a semiconductor sample such as a wafer by irradiating the sample with light of multiple wavelengths and detecting a surface potential change at that time by means of a noncontact probe placed near the sample.
However, this method is inconvenient in that it requires preparation of a relatively thick sample (generally 1 mm or thicker) for making an accurate measurement.
Meanwhile, as a method for evaluating the lifetime of minority carriers, a microwave photoconductivity decay method (μ-PCD method) is available. This method is a method which measures the lifetime of the carriers of a semiconductor sample by irradiating the sample with light and detecting a conductivity change at that time by means of a microwave. In particular, this method is often used for evaluation of contamination of silicon wafer.
However, this method is inconvenient in that it requires subjecting the surfaces of the sample to a special treatment such as an iodine ethanol treatment prior to the measurement. Consequently, a method of inhibiting surface recombination by hydrogen fluoride gas etching may be used in measuring the lifetime of the carriers of a semiconductor substrate by the μ-PCD method. However, this method requires a large-scale system for the gas etching.
Both of the above SPV method and μ-PCD method have a problem that measurement of two-dimensional carrier lifetime distribution within a sample wafer surface not only requires a stage for scanning the sample in the XY direction but also takes time because the measurement must be made while scanning the sample. The measurement time is as long as several tens of minutes per sample. Further, a low spatial resolution of several millimeters at most is also a problem.
Further, when a silicon substrate used in a solar cell or the like is to be evaluated by the above μ-PCD method or SPV method, the silicon substrate must be withdrawn from a production process of the solar cell or the like once, placed on the XY stage, placed in gas etching equipment if necessary, and evaluated over a long time. Therefore, this method cannot be used as in-line inspection of silicon substrates in a production process of solar cells or the like.
With a measurement method with spatial resolution of millimeter order such as the above μ-PCD method or SPV method, distribution of defects whose size is in micron order such as crystalline structural defects cannot be evaluated. That is, although low crystallinity of a silicon substrate can be evaluated macroscopically by these conventional methods, these methods cannot be used to evaluate what causes the low crystallinity of the silicon substrate microscopically. In this regard, it can be said that the conventional methods cannot provide information about improving the crystallinity of the silicon substrate sufficiently.
A photoluminescence (PL) imaging method developed in recent years is expected as a next-generation evaluation method that evaluates the characteristics of a silicon substrate with treated surfaces for production of a solar cell or the like in a short time of about 1 second or less and with a high spatial resolution of several tens of micrometers. In particular, since the method has high spatial resolution and crystalline defect distribution clearly emerges on a PL image as dark lines, it is expected to be very helpful as a measure for improving crystallinity. Further, it is known that PL intensity distribution obtained by the PL imaging method well-matches carrier lifetime distribution and diffusion length distribution obtained by the μ-PCD method and the SPV method.
However, the conventional PL imaging method has the following problem when a substrate with untreated surfaces is an object to be measured. That is, since photoluminescence to be detected is greatly influenced by surface recombination of the carriers in the substrate, accurate measurement becomes difficult and PL intensity lowers, so that measurement time becomes at least several tens of times longer than that when a substrate with treated surfaces is measured. This problem is particularly noticeable when the thickness of a substrate is small, and measurement is extremely difficult even when considerable time is spent therefor.
Patent Literature 1
Japanese Patent Laid-Open Publication No. 1993-129402.
Non-Patent Literature 1
T. Trupke, R. A. Bardos, M. C. Schubert and W. Warta, “Photoluminescence imaging of silicon wafers” Applied Physics Letters 89, 044107 (2006).
Non-Patent Literature 2
E. Yablonovitch, D. L. Allara, C. C. Chang, T. Gmitter and T. B. Bright, “Usually Low Surface-Recombination Velocity on Silicon and Germanium Surfaces” Physical Review Letter 57, p. 249 (1986).
In recent years, the thickness of a semiconductor (silicon) substrate for a solar cell has been reduced. This indicates that the influence of surface recombination of the carriers of a semiconductor substrate has become greater upon evaluation of the substrate. In this regard, the conventional PL imaging method is assumed to be unable to make accurate measurement easily because it is significantly influenced by recombination when a semiconductor substrate has small thickness or is not surface-treated. Further, since PL intensity lowers due to the influence of surface recombination, longer measurement time is required to obtain information sufficient to evaluate a semiconductor substrate by the conventional PL imaging method.
Further, along with a sharp increase in production of silicon solar cells, a technique that makes it possible to evaluate a large quantity of silicon substrates for solar cells in a short time has been demanded. Higher efficiency can be achieved if a method for evaluating a semiconductor substrate can be used as in-line inspection in a production process of solar cells or the like.
Further, a more accurate evaluation can be expected if a semiconductor substrate can be evaluated in a condition similar to a condition in which the semiconductor substrate is actually used in a solar cell or the like. For instance, in the conventional μ-PCD method in which the surfaces of a substrate are treated with hydrofluoric acid before the lifetime of the carriers of the substrate is measured, the sample substrate is subjected to an iodine ethanol treatment immediately after the substrate is taken out of hydrofluoric acid, so as to prevent degradation of the surface condition of the surface-treated substrate before the measurement. However, according to a finding by the present inventors, degradation of the surface condition of the surface-treated substrate starts rapidly from the moment the sample substrate is taken out of hydrofluoric acid for the surface treatment. This indicates that only substrates with some degree of deterioration in surface condition can be evaluated with the conventional method.
Further, when a substrate is evaluated by the PL imaging method and measurement must be made in a state of strong excitation because the measurement takes a long time, there is a possibility that characteristics in a condition different from a condition in which the substrate is actually used are seen because the substrate is evaluated in an environment different from actual light intensity at the time of operation of solar cell. In particular, when a substrate whose carrier lifetime is short is evaluated, measurement in strong excitation is unavoidable with the conventional method due to low signal strength, and this problem is noticeable.
Therefore, an object of the present invention is to provide a method for evaluating a semiconductor substrate that can evaluate even a thin semiconductor substrate or a substrate with untreated surfaces by inhibiting surface recombination of the carriers of the substrate.
Another object of the present invention is to provide a method for evaluating a semiconductor substrate that can evaluate a large quantity of semiconductor substrates for solar cells in a short time and can be used as in-line inspection in a production process of solar cells or the like.
Still another object of the present invention is to provide a method for evaluating a semiconductor substrate that makes it possible to evaluate a semiconductor substrate in a condition similar to a condition in which the semiconductor substrate is actually used in a solar cell or the like, from the viewpoints of the surface condition of the substrate to be evaluated and the intensity of light to be radiated.